This PFC main goal is to introduce new approaches on the chromatic dispersion measurement
field, based on the big range of possibilities the setup of a general standard RF-tone
modulation chromatic modulation method provides, and to show its good performance
pointing towards a real-time on-line monitoring system for optical communication networks.
The project’s objectives are defined considering two well-delimited stages.
First, we will study some standard RF-tone-addition techniques for measuring chromatic
dispersion, specifically the Modulation Phase Shift Method (MPSM) [2] and the Peucheret`s
Method [3]. We will analyze their operating principles, recognize all the variables involved in
their basic configurations and evaluate their performances under different measurement
conditions.
We will also study the implications of real-time on-line monitoring of chromatic dispersion in
optical networks. We have to consider that the test signal has to travel together with the data;
therefore, it is a priority to keep the optical carrier unaltered in the transmission and reception
procedures.
This background will help us to identify the main drawbacks of both methods which motivate
the proposal of a new improved technique based on a similar mathematical basis but with
better performance in terms of accuracy and cost trade-off.
The general features of this new approach will be exposed on a basic setup designed for a
laboratory environment, so that we can contrast it with the conventional techniques. This
method dubbed Asymmetric Modulation Bias-Controlled Method (ABCM) will focus on RF modulated signal amplitude and will take advantage of its direct relation with chromatic
dispersion.
One of the basic building blocks of these standard methods is the device that imposes the RF
pure-tone modulation to the optical signal, namely the Mach-Zehnder interferometric
modulator usually in the conventional push-pull configuration and biased at the quadrature
point. In the context of the new improved CD measurement methods, we will observe how the
Mach-Zehnder modulator Bias Voltage concept gains relevance; becoming the main variable to
be handled by the use of a dual drive Mach-Zehnder modulator in asymmetric configuration.
Finally, we will analyze this ABCM method performance while some fixed parameters (RF
Frequency, Nominal Dispersion,
resolution) take different values in order to find out the
optimum operating conditions.
The problem when trying to apply the ABCM to the real-time on-line monitoring of optical
networks is that it relies in the eventual cancellation of the optical carrier which in a network
monitoring application is shared with the data and it is essential for a proper data recovery.
We must find an alternative where this optical carrier cancellation is not essential for the
monitoring function and that would be the ABCM-SC (SC for suppressed carrier)
Therefore, on a second stage, we will focus on giving this new perspective about dispersion
measurement a direct application in optical communications field. We will restructure the
ABCM into a practical dispersion monitoring system for optical communication networks. This
improved monitoring technique will be based on a proof-of-concept study (no real data
transmission considered) to evaluate the method’s performance in terms of accuracy,
robustness and adaptability, building the basis for data transmission experiments in future
projects.
An important aspect to take into account will be the way we carry out the RF tone addition
procedure without altering the optical carrier (transmitted data). To accomplish this
requirement we will use a Bessel function analysis to achieve a carrier-suppressed modulation
of the RF tone, which introduces another important handling parameter: the RF Tone
Amplitude.
We will also be concerned about isolating the emitter part (where data is transmitted) from
the monitoring point (where dispersion is measured), but at the same time complementing
each other to operate in a real-time situation.
Finally, we will study the requirement of including the second RF harmonic detection together
with the first harmonic as it adjusts better to a real-time monitoring system and increases the
accuracy level in chromatic dispersion measurement.